During operation in the heating mode, the outdoor heat exchanger of an air source heat pump provides means to vaporize a refrigerant liquid by heat transfer from air flowing through the heat exchanger. Efficient operation of the system requires that sufficient heat be transferred from the air flowing through the outdoor heat exchanger to maintain adequate capacity to meet the heating demand in a comfort zone.
If the outdoor ambient air temperature is less than approximately 32.degree. F., frost and ice may accumulate on the heat exchanger, blocking air flow therethrough to such an extent that its capacity for heat transfer is reduced below that required to meet the heating demand in the comfort zone. It is therefore common practice to defrost the outdoor heat exchanger, melting the accumulated frost and ice, to prevent an unacceptable level of heat transfer degradation.
One of the simplest methods of preventing excessive frost accumulation on the outdoor heat exchangers is to initiate a defrost cycle at timed intervals. A control for such a time-based defrost method should provide for a relatively long interval between defrost cycles at low ambient air temperatures. At outdoor ambient air temperatures less than 0.degree. F., the relative humidity is usually close to 100%; yet, at these temperatures, the volume of water vapor per unit volume of air is relatively low. As a result, it takes longer for frost to accumulate on an outdoor heat exchanger than it does at higher ambient air temperatures. Since the defrost cycle typically wastes energy, it should not be implemented more often, nor for a longer period than necessary to maintain the required capacity. It is thus preferable to initiate a defrost cycle only after sufficient ice and frost have formed on the outdoor heat exchanger to cause a problem in meeting the heating demand.
There are numerous techniques in the prior art for sensing an accumulation of frost on the outdoor heat exchanger, as for example, detecting a reduction in air flow through the heat exchanger, or the scattering of a reflected light beam by frost crystals. Such techniques provide little more than an indication that frost has formed and that heat transfer is at least partially degraded thereby. More sophisticated techniques provide means for sensing the extent of heat transfer degradation due to frost accumulation, e.g., by the relationship of the outdoor ambient air temperature and the outdoor heat exchanger temperature.
If the indoor or comfort zone setpoint temperature remains constant, such techniques may provide efficient, reliable defrost cycle operation. However, if the setpoint temperature in a comfort zone is changed significantly, as for example due to night setback, the prior art defrost controls do not provide means to compensate for the change in the minimum required heating capacity to meet the demand. As a result, the defrost cycle is not controlled with optimum efficiency and reliability.
It is therefore an object of this invention to provide a method and control for defrosting an outdoor heat exchanger, which optimizes efficiency and reliability of the temperature conditioning system as a function of the comfort zone temperature.
Another object of this invention is to control the defrost cycle in a manner which compensates for a change in the required heating capacity due to a change in the comfort zone setpoint temperature.
It is a further object of this invention to terminate the defrost cycle as soon as a sufficient quantity of ice and frost accumulated on the outdoor heat exchanger have melted to resume reliable and efficient operation of the heat pump system.
A still further object of this invention is to provide means to initiate a deferred defrost cycle, if the relative water vapor content of the outdoor ambient air is so low that frost and ice accumulate on the outdoor heat exchanger very slowly.
These and other objects of the subject invention will become apparent from the description which follows and by reference to the attached drawings.